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Ganymede
subsurface oceans Artist's cut-away representation of the internal structure of Ganymede. Layers drawn to scale. In the 1970s, NASA scientists first suspected that Ganymede has a thick ocean between two layers of ice, one on the surface and one beneath a liquid ocean and atop the rocky mantle.417616465 In the 1990s, NASA's Galileo mission flew by Ganymede, confirming the moon's sub-surface ocean. An analysis published in 2014, taking into account the realistic thermodynamics for water and effects of salt, suggests that Ganymede might have a stack of several ocean layers separated by different phases of ice, with the lowest liquid layer adjacent to the rocky mantle.17181966 Water–rock contact may be an important factor in the origin of life.17 The analysis also notes that the extreme depths involved (~800 km to the rocky "seafloor") mean that temperatures at the bottom of a convective (adiabatic) ocean can be up to 40 K higher than those at the ice–water interface. In March 2015, scientists reported that measurements with the Hubble Space Telescope of how the aurorae moved over Ganymede's surface suggest it has a subsurface ocean. A large salt-water ocean affects Ganymede's magnetic field, and consequently, its aurora.15666768 The evidence suggests that Ganymede's oceans might be the largest in the entire Solar System.69 There is some speculation on the potential habitability of Ganymede's ocean.6570 Core The existence of a liquid, iron–nickel-rich core62 provides a natural explanation for the intrinsic magnetic field of Ganymede detected by Galileo spacecraft.71 The convection in the liquid iron, which has high electrical conductivity, is the most reasonable model of magnetic field generation.20 The density of the core is 5.5–6 g/cm3 and the silicate mantle is 3.4–3.6 g/cm3.39616371 The radius of this core may be up to 500 km.71 The temperature in the core of Ganymede is probably 1500–1700 K and pressure up to 10 GPa (99,000 atm).6171 Atmosphere and ionosphere In 1972, a team of Indian, British and American astronomers working in Java (Indonesia) and Kavalur (India) claimed that they had detected a thin atmosphere during an occultation, when it and Jupiter passed in front of a star.72 They estimated that the surface pressure was around 0.1 Pa.72 However, in 1979, Voyager 1 observed an occultation of the star κ Centauri during its flyby of Jupiter, with differing results.73 The occultation measurements were conducted in the far-ultraviolet spectrum at wavelengths shorter than 200 nm, which were much more sensitive to the presence of gases than the 1972 measurements made in the visible spectrum. No atmosphere was revealed by the Voyager data. The upper limit on the surface particle number density was found to be 1.5×109 cm−3, which corresponds to a surface pressure of less than 2.5 µPa.73 The latter value is almost five orders of magnitude less than the 1972 estimate.73 False-color temperature map of Ganymede Despite the Voyager data, evidence for a tenuous oxygen atmosphere (exosphere) on Ganymede, very similar to the one found on Europa, was found by the Hubble Space Telescope (HST) in 1995.1074 HST actually observed airglow of atomic oxygen in the far-ultraviolet at the wavelengths 130.4 nm and 135.6 nm. Such an airglow is excited when molecular oxygen is dissociated by electron impacts,10 which is evidence of a significant neutral atmosphere composed predominantly of O2 molecules. The surface number density probably lies in the (1.2–7)×108 cm−3 range, corresponding to the surface pressure of 0.2–1.2 µPa.10i These values are in agreement with the Voyager's upper limit set in 1981. The oxygen is not evidence of life; it is thought to be produced when water ice on Ganymede's surface is split into hydrogen and oxygen by radiation, with the hydrogen then being more rapidly lost due to its low atomic mass.74 The airglow observed over Ganymede is not spatially homogeneous like that over Europa. HST observed two bright spots located in the northern and southern hemispheres, near ± 50° latitude, which is exactly the boundary between the open and closed field lines of the Ganymedian magnetosphere (see below).75 The bright spots are probably polar auroras, caused by plasma precipitation along the open field lines.76 The existence of a neutral atmosphere implies that an ionosphere should exist, because oxygen molecules are ionized by the impacts of the energetic electrons coming from the magnetosphere77 and by solar EUV radiation.21 However, the nature of the Ganymedian ionosphere is as controversial as the nature of the atmosphere. Some Galileo measurements found an elevated electron density near Ganymede, suggesting an ionosphere, whereas others failed to detect anything.21 The electron density near the surface is estimated by different sources to lie in the range 400–2,500 cm−3.21 As of 2008, the parameters of the ionosphere of Ganymede are not well constrained. Additional evidence of the oxygen atmosphere comes from spectral detection of gases trapped in the ice at the surface of Ganymede. The detection of ozone (O3) bands was announced in 1996.78 In 1997 spectroscopic analysis revealed the dimer (or diatomic) absorption features of molecular oxygen. Such an absorption can arise only if the oxygen is in a dense phase. The best candidate is molecular oxygen trapped in ice. The depth of the dimer absorption bands depends on latitude and longitude, rather than on surface albedo—they tend to decrease with increasing latitude on Ganymede, whereas O3 shows an opposite trend.79 Laboratory work has found that O2 would not cluster or bubble but dissolve in ice at Ganymede's relatively warm surface temperature of 100 K (−173.15 °C).80 A search for sodium in the atmosphere, just after such a finding on Europa, turned up nothing in 1997. Sodium is at least 13 times less abundant around Ganymede than around Europa, possibly because of a relative deficiency at the surface or because the magnetosphere fends off energetic particles.81 Another minor constituent of the Ganymedian atmosphere is atomic hydrogen. Hydrogen atoms were observed as far as 3,000 km from Ganymede's surface. Their density on the surface is about 1.5×104 cm−3.82 Magnetosphere Magnetic field of the Jovian satellite Ganymede, which is embedded into the magnetosphere of Jupiter. Closed field lines are marked with green color. The Galileo craft made six close flybys of Ganymede from 1995–2000 (G1, G2, G7, G8, G28 and G29)20 and discovered that Ganymede has a permanent (intrinsic) magnetic moment independent of the Jovian magnetic field.83 The value of the moment is about 1.3 × 1013 T·m3,20 which is three times larger than the magnetic moment of Mercury. The magnetic dipole is tilted with respect to the rotational axis of Ganymede by 176°, which means that it is directed against the Jovian magnetic moment.20 Its north pole lies below the orbital plane. The dipole magnetic field created by this permanent moment has a strength of 719 ± 2 nT at Ganymede's equator,20 which should be compared with the Jovian magnetic field at the distance of Ganymede—about 120 nT.83 The equatorial field of Ganymede is directed against the Jovian field, meaning reconnection is possible. The intrinsic field strength at the poles is two times that at the equator—1440 nT.20 Aurorae on Ganymede—auroral belt shifting may indicate a subsurface saline ocean. The permanent magnetic moment carves a part of space around Ganymede, creating a tiny magnetosphere embedded inside that of Jupiter; it is the only moon in the Solar System known to possess the feature.83 Its diameter is 4–5 ''R''G (''R''G = 2,631.2 km).84 The Ganymedian magnetosphere has a region of closed field lines located below 30° latitude, where charged particles (electrons and ions) are trapped, creating a kind of radiation belt.84 The main ion species in the magnetosphere is single ionized oxygen—O+21—which fits well with Ganymede's tenuous oxygen atmosphere. In the polar cap regions, at latitudes higher than 30°, magnetic field lines are open, connecting Ganymede with Jupiter's ionosphere.84 In these areas, the energetic (tens and hundreds of kiloelectronvolt) electrons and ions have been detected,77 which may cause the auroras observed around the Ganymedian poles.75 In addition, heavy ions precipitate continuously on Ganymede's polar surface, sputtering and darkening the ice.77 The interaction between the Ganymedian magnetosphere and Jovian plasma is in many respects similar to that of the solar wind and Earth's magnetosphere.8485 The plasma co-rotating with Jupiter impinges on the trailing side of the Ganymedian magnetosphere much like the solar wind impinges on the Earth's magnetosphere. The main difference is the speed of plasma flow—supersonic in the case of Earth and subsonic in the case of Ganymede. Because of the subsonic flow, there is no bow shock off the trailing hemisphere of Ganymede.85 In addition to the intrinsic magnetic moment, Ganymede has an induced dipole magnetic field.20 Its existence is connected with the variation of the Jovian magnetic field near Ganymede. The induced moment is directed radially to or from Jupiter following the direction of the varying part of the planetary magnetic field. The induced magnetic moment is an order of magnitude weaker than the intrinsic one. The field strength of the induced field at the magnetic equator is about 60 nT—half of that of the ambient Jovian field.20 The induced magnetic field of Ganymede is similar to those of Callisto and Europa, indicating that Ganymede also has a subsurface water ocean with a high electrical conductivity.20 Given that Ganymede is completely differentiated and has a metallic core,471 its intrinsic magnetic field is probably generated in a similar fashion to the Earth's: as a result of conducting material moving in the interior.2071 The magnetic field detected around Ganymede is likely to be caused by compositional convection in the core,71 if the magnetic field is the product of dynamo action, or magnetoconvection.2086 Despite the presence of an iron core, Ganymede's magnetosphere remains enigmatic, particularly given that similar bodies lack the feature.4 Some research has suggested that, given its relatively small size, the core ought to have sufficiently cooled to the point where fluid motions, hence a magnetic field would not be sustained. One explanation is that the same orbital resonances proposed to have disrupted the surface also allowed the magnetic field to persist: with Ganymede's eccentricity pumped and tidal heating of the mantle increased during such resonances, reducing heat flow from the core, leaving it fluid and convective.52 Another explanation is a remnant magnetization of silicate rocks in the mantle, which is possible if the satellite had a more significant dynamo-generated field in the past.4 Kingdom of Abzu, Categoria:Abzu Domain Categoria:Planets Categoria:Milky Way Planets